If the Islanders fall short of the playoffs, this is one game they will look back on with regret.

Defenseman Sergei Gonchar’s go-ahead goal with a minute left was part of a four-goal, third-period outburst by the Senators, who rallied from a two-goal deficit and earned a 5-3 victory over the Islanders Tuesday night at the Coliseum.

It was the second straight game in which the Islanders — three points out of a playoff spot in the Eastern Conference — squandered a third-period lead. They led Florida 3-0 on Saturday before the Panthers tied it. But Casey Cizikas scored the winner for New York with seven minutes left.

“I’m just glad we got the two points and kept pace in the conference,” Gonchar said.

Gonchar’s second goal of the season came on a blast from the point to make it 4-3, and Guillaume Latendresse added an empty-net goal to seal the win. Jakob Silfverberg and Zack Smith scored in a 68-second span early in the third period to tie the game after the Islanders led 3-1.

Ottawa (16-8-6) has won three straight.

Josh Bailey scored the go-ahead goal 14 seconds into the second period, and Lubomir Visnovsky extended the Islanders’ lead to two goals.

Keith Aucoin appeared to give the Islanders a 4-3 lead at 7:22, but the goal was disallowed because he kicked the puck past Bishop.

Bishop also stopped Aucoin, who scored the Islanders’ first goal, on a sharp-angle shot from the left corner late in the third.

Ottawa’s Matt Kassian opened the scoring at 6:23 with his first goal of the season and first with the Senators when he redirected Gonchar’s slap shot past Nabokov.

Aucoin tied it with a wrist shot past Bishop at 17:53 of the first, netting his fifth of the season.

Bailey’s third goal, on a backhand that skittered past Bishop, put the Islanders up 2-1. It was the 50th career goal for Bailey, a first-round draft pick by the Islanders in 2008. Visnovsky put the Islanders ahead 3-1 with a power-play goal at 11:46 of the second.

The Islanders (13-13-3) began a four-game homestand after road wins over Tampa Bay and Florida. They are just 5-9-2 at home.